Hint of writing fluid found on heather (7)
I believe the answer is:
inkling
'hint of' is the definition.
(I've seen this before)
'fluid found on heather' is the wordplay.
'fluid' becomes 'ink' (ink is a kind of fluid**).
'found on' means one lot of letters go next to another.
'heather' becomes 'ling' ('ling' can be a synonym of 'heather').
'ink'+'ling'='INKLING'
'writing' acts as a link.
(Other definitions for inkling that I've seen before include "Slight hint or suspicion" , "Glimmer of understanding" , "Faint notion" , "Conception" , "Linking with suspicion or hint" .)